1. Field of the Invention
The present invention relates to a novel process for dyeing melt spun synthetic polymeric filaments. The process of the present invention is used to produce a highly oriented filament having a uniform dyed cross-section.
2. Prior Art
It is well known in the art that dyeing melt spun synthetic polymer filament is affected by numerous variables including drawing and heat treating of the filaments as well as the selected dyeing process. Each of these processes are interrelated and necessary for production of melt spun synthetic polymeric filaments.
Generally it is known to produce melt spun synthetic polymeric filaments by the steps of melt spinning the polymer to form filaments, drawing the filaments to obtain desired tensile and thermal properties, annealing the filaments, crimping and drying the filaments. It is well known that annealing of the filaments may be combined with the drying the filaments at elevated temperatures.
Drawing occurs when the melt spun filaments are elongated. In particular, drawing is defined as the stretching of continuous filament yarn or tow to increase the average axial alignment of the polymer molecules of the filament resulting in improved tensile and thermal properties. Such yarns are termed drawn or tensilized yarns.
During the drawing process the polymer molecules in the filament or fibers become oriented and more closely packed generally referred to as orientation in the filaments. This increases the specific strength and modulus of the filaments. The actual amount of drawing necessary is determined by the amount of orientation developed in the spinning process and also by the level of tenacity desired in the final product. For example, in U.S. Pat. No. 3,216,187, Chantry et al describe a polyester process designed to minimize orientation in the melt spun polyester yarn and then the use of high draw ratios, e.g. 6.7:1, to obtain a very strong yarn for use in, for example, reinforcing automobile tires. Conversely, U.S. Pat. No. 4,134,882 describes spinning at very high stresses to maximize spun yarn birefringence or spun molecular orientation and thereby minimize or eliminate the drawing step.
Generally, drawing is done at a temperature of the tow from about 60.degree. C. to about 80.degree. C. i.e. at temperatures near the glass transition temperature of the polymer.
Further improvements in yarn strength and shrinkage characteristics result from annealing the drawn yarns at temperatures significantly above their glass transition temperatures. Generally, annealing of polyester filaments occurs immediately subsequent to the drawing process at temperatures from about 160.degree. C.-200.degree. C. for a short period of time. Annealing the filament further increases the density of the filament and stabilizes it against subsequent shrinkage when subjected to heat. Such heat treatment is generally referred to as thermal crystallization of the filament.
Two forms of annealing are commonly practiced. If high tensile properties are desired, the annealing is carried out at a predetermined fixed length. In continuous processes, this is usually accomplished by using a series of rolls operating at fixed speeds. The heat may be applied either by heating the roll surfaces or installing another heating device such as a hot plate or a hot air oven between the rolls.
A second form, annealing in a free shrink condition will, in general, result in lower final shrinkage values than fixed length annealing but will also reduce the tenacity and modulus. In commercial practice, free to shrink annealing is usually accomplished by loosely depositing the filaments onto a conveyor belt which passes through a hot air oven.
As produced, melt spun filaments and fibers are smooth, essentially one dimensional and uniform in character. In many applications this smoothness and one dimensional character are undesirable. Consequently, the filaments are subjected to a bulking process. Bulking may be accomplished, for example, by steam or air jets, by stuffer boxes, by edge treatments, or by false twist texturing. Texturing is often done at elevated temperatures to provide more durable bulk.
Subsequent to bulking, the filaments are dried. However, it is known in the art that the annealing stage may be replaced with heat setting in the drying stage. If annealing is omitted between drawing and bulking, then the filaments are generally dried and heat set at temperatures greater than the glass transition temperature for a time of 5 minutes or more.
Dyeing processes are based on the specific melt spun synthetic filament. Dyeing may be accomplished by application of the dye to the spun filaments or incorporation of the dye into the polymer melt prior to spinning. The process includes application of the dye plus suitable processes to affix the dye to the synthetic filament.
Polyesters are most often dyed with disperse dyes, also known as acetate dyes. These dyes are virtually insoluble in water but may be dispersed as very fine particles. They are sold in finely divided form together with a dispersing agent and fillers. Typically the actual dye represents only about 20% of the commercial dye powder. In commercial practice, these dyes are applied to the fibers by aqueous dispersions. The dispersing agents and fillers are then washed off and discarded.
The dyeing process contains several steps and generally requires several hours to complete. The steps typically include pre scour, actual dyeing and post scouring steps. Generally, not all of the dye enters the fiber. Some remains in the dye bath and some remains on the fiber surface. Several washing and scouring steps may be required to remove surface dye and the dye assist chemicals. Each step in the dyeing process generates significant quantities of waste water.
The dyeing process may be enhanced by the use of pressure or high temperature dyeing. The dyeing process may be quickened by higher dyeing temperatures. High temperature dyeing is accomplished using super-atmospheric pressures to allow dyeing temperatures, for example, for polyester at about 130.degree. C. Pressure dyeing requires specialized pressure containment vessels and adds significantly to the energy costs of dyeing.
It is also known to produce colored filament by adding selected thermally stable pigments directly to the polymer melt and spinning filaments which are already colored or pigmented with the thermally stable pigments. This process is variously called melt coloration, spin dyeing or solution dyeing. It is now used on a large scale for filaments. Although care must be taken to minimize pigment thermal degradation and off gassing during spinning, mass coloration is more suitable to the environment than traditional dyeing.
However, its primary disadvantage is that large batches of filaments have to be produced and the range of colors which can be commercially available is limited. Also, color matching and color control in the melt spun coloration can be extremely tedious and the selection of acceptable pigments is much smaller than the selection of disperse dyes.
Various dyeing processes used before the drawing of the filament are known. For example, U.S. Pat. No. 3,241,906 discloses a dyeing step prior to the drawing process so that both coloring and strengthening of the filaments can be performed in one continuous operation. Specifically, the process is directed to applying a dye dispersed, without dispersing agents or other diluents, in an organic, substantially nonaqueous, hot solvent to undrawn melt spun filaments under conditions such that the penetration of the dye into the filaments is 60% of the cross-sectional area or more. The dye is applied at temperatures up to 150.degree. C. for a short time, preferably less than 30 seconds, using an organic solvent which dissolves the dye but does not cause embrittlement of the filaments. This is followed by drawing of the filaments 3 to 6 times their length.
Combining conventional aqueous dyeing systems with conventional drawing systems has so far not proven to be commercially viable. The primary reason is that dyeing is a diffusion controlled process and sufficient times are not available during drawing to permit completion of the diffusion. Attempts have been made to accelerate diffusion. British Patent Specification 1,094,725 discloses a process for the continuous dyeing of polyester filaments wherein a dispersion of dyestuff is applied to the stretched or unstretched filaments which are then drawn if undrawn, dried and allowed to shrink by at least 10% at a temperature within 50.degree. C. of its melting point. After the dye is applied, the material is dried at 50.degree. to 80.degree. C. and then shrunk at a much higher temperature. The rate of dye diffusion is considerably enhanced during the high temperature relaxation step and an acceptable level of dye penetration is achieved. After shrinkage, the yarn may be redrawn to re-establish the desired level of strength.
This process has the disadvantage that several additional, non-standard steps have been added to the drawing process requiring new drawline designs. In particular, in order to achieve the high shrinkages required by this process the draw tension must be released before the yarn is heated. If the heat is applied first the yarn will anneal and the requisite shrinkages can not be obtained. Traditional drawing equipment is not configured with this capability. Also, several post draw washing and scouring steps are required to remove excess dye and dye assist chemicals clearly showing that the dyeing was still not complete.
A process to uniformly dye synthetic fibers is disclosed in U.S. Pat. No. 2,663,612. A process includes impregnating the fiber by padding or printing with an aqueous suspension of the dye, drying the impregnated fiber and then giving it a dry heat treatment at a temperature of between 180.degree. C. and 230.degree. C. for a brief interval of time, 5 to 60 seconds.
It is an aim or aspect of the present invention to provide a process for continuous dyeing of synthetic filaments resulting in a drawn filament having essentially 100% cross-sectional dye uptake.